The present invention relates to optical scanners and more particularly to optical scanners with beam directing holograms at the scanner window for redirecting impinging light beams as those beams leave the scanner enclosure through the window.
Fixed head optical scanners are commonly used with processor controlled point of sale systems in retail stores and supermarkets to identify products being processed. With such scanners, a label-carrying product is moved across a transparent scanner window which is normally in the top surface of a stationary scanner enclosure. The scanner enclosure may, for example, be built into a checkout stand in a supermarket. An optical system within the scanner deflects a laser beam to produce one or more scan lines which sweep through a region above the scanner window. Light reflected from the product is detected. Using an electrical signal proportional to the reflected light, potential label candidates are selected and decoded by electronic circuits or microprocessors within the scanner enclosure.
A scanner should be able to detect and successfully decode almost all labels on the first pass of each labeled product over the scanner window. If a label cannot be detected and successfully decoded on the first pass, the product must be moved across the scanner window one or more times. The additional passes or re-scans are obviously time consuming and thus harmful to productivity.
To make an operator as productive as possible, few constraints should be placed on the way a product is held as that product is moved across the scanner window. This means, of course, that the label on the product must be considered as being randomly oriented relative to the scanner. To increase the chances that a randomly oriented label will be properly scanned during a single pass, multi-line scan patterns are used in which the lines intersect each other at angles. The scan patterns are generated by directing a laser beam at a rotating mirror wheel or through a rotating holographic disc to cause the laser beam to be deflected along scan lines. Fixed arrangements of mirrors are used to change the beam paths before the beam leaves the enclosure through the scanner window. Known scan patterns include non-parallel or intersecting lines and/or parallel lines displaced in space from one another.
Scan patterns have been developed which are quite effective in reading bar coded labels on leading surfaces or bottom surfaces of products being scanned. If the labels are on side surfaces or trailing surfaces, however, the same scan patterns are not as effective since the scanning beams generated by conventional scanners are generally directed toward the approach end of the scanner enclosure. In fact, many scanners are incapable of reading a label appearing on a trailing or side surface of a product as it passes over the scanner window.
While it may be possible to achieve side scanning or rear scanning lines through the use of additional beam folding mirrors within the scanner enclosure, this is not a desirable solution to the problem. Designing, assembling and aligning such mirrors is relatively costly. Moreover, space within the enclosure is limited, making it difficult to position and mount such mirrors without encroaching on space required for other components of the scanner.